This invention relates to improvements in a piston for an internal combustion engine, more particularly to a piston whose sliding section is coated with a hard carbon thin film such as a diamond-like carbon thin film which exhibits a highly excellent low friction characteristics particularly in presence of a specified lubricating oil (composition).
Global environmental problems such as global warming and ozone layer destruction are coming to the fore. As it has been said that the global warming is significantly affected by CO2 emission, the reduction of CO2 emission, notably the setting of CO2 emission standards, has become a big concern to each country. One of challenges to reduce CO2 emission is to improve vehicle fuel efficiency that depends on the performance of engine sliding members and a lubricating oil applied thereto. There are the following approaches to improving vehicle fuel efficiency: (1) lowering the viscosity of the lubricating oil, thereby reducing viscous resistance in hydrodynamic lubrication regions and agitation resistance in the engine; and (2) to adding a suitable friction modifier and other additives into the lubricating oil so as to reduce friction losses under the conditions of mixed lubrication and boundary lubrication.
In view of the foregoing, many studies have been made on various friction modifiers including organomolybdenum compounds, such as molybdenum ditiocarbamate (MoDTC) and molybdenum dithiophosphate (MoDTP). As a result of the studies, it has been found that the lubricating oil containing an organomolybdenum compound successfully produces a friction reducing effect on the steel sliding members in the early stages of use.